1. Field of invention
This invention relates to an improved chemical system for the enhancement and maintenance of cementaceous and composite asphalt building materials.
2. Description of the Prior Art
Cement tile and asphalt shingles have historically dominated the residential roofing market. Despite their success, the unattractive discoloration encountered throughout the United States and Canada particularly in moist temperate climates is becoming increasingly unacceptable. While the approach among respective manufacturers differ, the consensus in the cement tile industry has deferred the problem to the after market roof cleaning industry which has flourished in providing the routine service required to maintain appearance. High pressure water systems incorporating chlorine bleach are typically used. In warm humid climates typical in the Gulf States the service is required annually. Roof painting where permitted can postpone reoccurrence approximately two fold. In contrast, the asphalt shingle industry cautions against the use of high pressure washing systems as the process may remove granules which will shorten roof life. A gentle application of dilute aqueous chlorine bleach and trisodium phosphate from a ladder or walkboards is suggested by the Asphalt Roofing Manufacturers Association to avoid roof damage. The effectiveness of such cleaning is only temporary and there is little evidence of successful practitioners serving the residential asphalt shingle after-market. However, manufacturers offer several types of algae resistant products which have met with limited success.
The state-of-the-art technology for algae resistant asphalt shingles centers on a source of microbicide which can be released as a result of mechanical and/or chemical weathering. Of commercial significance, are shingles incorporating a percentage of zinc metal or zinc oxide coated granules as the inhibitor source. A second generation incorporating copper oxide as an inhibitor source of relatively higher toxicity has recently been introduced. The release of soluble metallic salts from these sources is typically via adsorption of carbon and sulfur dioxides in presence of moisture yielding acidic reactants. The residence time of soluble metal salts released from these products can be quite limited being readily removed by rain. However, slow release of inhibitor from these relatively inert sources is projected to continue over extended periods of time. Clearly, environmental conditions as they effect release and residence time on roof surface are major variables in this mechanism of inhibition. The effectiveness of the technology can only be improved with increased quantities of source material exhibiting higher levels of toxicity to counteract the transient nature of the active chemical inhibitors.
The present industry approach appears well founded on the observed inhibition which develops on the trailing surfaces below metal vents and stand pipes installed with roofing systems. While this potential source of tin, copper and zinc microbicide in the form of soluble metal salts is obvious, the mechanism by which inhibition is established may not be. A priori, released microbicide from any source must have a finite residence time in order to be effective. For this purpose, the exchange and chelation of polyvalent metallic ions with organic and inorganic receptor sites that develop on weathered asphalt and granule surfaces is well documented. Carboxylic acid end groups represent the highest oxidation state of asphalt surface while hydrous oxide surface functionality results from granule weathering. As electron donor sites, both interact with polyvalent metal cations to increase their residence time on weathered roof surfaces. Uncontrolled early release of such inhibitors contributes both to their fugitive nature and the substantial chemical inefficiencies inherent in the present state-of-the-art. Further compromises result from the complexities of asphalt shingle manufacture involving distribution and adhesion by partial embedment as the only means of securing granule inhibitor sources.